[tt] simulating event horizonts

[Eugen Leitl]

http://www.universetoday.com/2008/02/13/synthetic-black-hole-event-horizon-created-in-uk-laboratory/

Synthetic Black Hole Event Horizon Created in UK Laboratory

Written by Ian O'Neill

An artists impression of a black hole

Researchers at St. Andrews University, Scotland, claim to have found a way to
simulate an event horizon of a black hole - not through a new cosmic
observation technique, and not by a high powered supercomputer… but in the
laboratory. Using lasers, a length of optical fiber and depending on some
bizarre quantum mechanics, a "singularity" may be created to alter a laser's
wavelength, synthesizing the effects of an event horizon. If this experiment
can produce an event horizon, the theoretical phenomenon of Hawking Radiation
may be tested, perhaps giving Stephen Hawking the best chance yet of winning
the Nobel Prize.

So how do you create a black hole? In the cosmos, black holes are created by
the collapse of massive stars. The mass of the star collapses down to a
single point (after running out of fuel and undergoing a supernova) due to
the massive gravitational forces acting on the body. Should the star exceed a
certain mass "limit" (i.e. the Chandrasekhar limit - a maximum at which the
mass of a star cannot support its structure against gravity), it will
collapse into a discrete point (a singularity). Space-time will be so warped
that all local energy (matter and radiation) will fall into the singularity.
The distance from the singularity at which even light cannot escape the
gravitational pull is known as the event horizon. High energy particle
collisions by cosmic rays impacting the upper atmosphere might produce
micro-black holes (MBHs). The Large Hadron Collider (at CERN, near Geneva,
Switzerland) may also be capable of producing collisions energetic enough to
create MBHs. Interestingly, if the LHC can produce MBHs, Stephen Hawking's
theory of "Hawking Radiation" may be proven should the MBHs created evaporate
almost instantly.

Hawking predicts that black holes emit radiation. This theory is paradoxical,
as no radiation can escape the event horizon of a black hole. However,
Hawking theorizes that due to a quirk in quantum dynamics, black holes can
produce radiation.  The principal of Hawking Radiation (source:
http://library.thinkquest.org) Put very simply, the Universe allows particles
to be created within a vacuum, "borrowing" energy from their surroundings. To
conserve the energy balance, the particle and its anti-particle can only live
for a short time, returning the borrowed energy very quickly by annihilating
with each other. So long as they pop in and out of existence within a quantum
time limit, they are considered to be "virtual particles". Creation to
annihilation has net zero energy.

However, the situation changes if this particle pair is generated at or near
an event horizon of a black hole. If one of the virtual pair falls into the
black hole, and its partner is ejected away from the event horizon, they
cannot annihilate. Both virtual particles will become "real", allowing the
escaping particle to carry energy and mass away from the black hole (the
trapped particle can be considered to have negative mass, thus reducing the
mass of the black hole). This is how Hawking radiation predicts "evaporating"
black holes, as mass is lost to this quantum quirk at the event horizon.
Hawking predicts that black holes will gradually evaporate and disappear,
plus this effect will be most prominent for small black holes and MBHs.

So… back to our St. Andrews laboratory…

Prof Ulf Leonhardt is hoping to create the conditions of a black hole event
horizon by using laser pulses, possibly creating the first direct experiment
to test Hawking radiation. Leonhardt is an expert in "quantum catastrophes",
the point at which wave physics breaks down, creating a singularity. In the
recent "Cosmology Meets Condensed Matter" meeting in London, Leonhardt's team
announced their method to simulate one of the key components of the event
horizon environment.

Light travels through materials at different velocities, depending on their
wave properties. The St. Andrews group use two laser beams, one slow, one
fast. First, a slow propagating pulse is fired down the optical fiber,
followed by a faster pulse. The faster pulse should "catch up" with the
slower pulse. However, as the slow pulse passes through the medium, it alters
the optical properties of the fiber, causing the fast pulse to slow in its
wake. This is what happens to light as it tries to escape from the event
horizon - it is slowed down so much that it becomes "trapped".

    "We show by theoretical calculations that such a system is capable of
probing the quantum effects of horizons, in particular Hawking radiation." -